Not Applicable
The present invention relates to corrugated structured packing and methods for installing such packing in an exchange column to provide optimum performance. The structured packing has particular application in exchange columns, especially in cryogenic air separation processes, although it also may be used in other heat and/or mass transfer processes that can utilize structured packing.
The term, xe2x80x9ccolumnxe2x80x9d, as used herein, means a distillation or fractionation column or zone, ie., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term xe2x80x9ccolumn sectionxe2x80x9d (or xe2x80x9csectionxe2x80x9d) means a zone in a column filling the column diameter. The top or bottom of a particular section or zone ends at the liquid and vapor distributors respectively.
The term xe2x80x9cpackingxe2x80x9d means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are xe2x80x9crandomxe2x80x9d and xe2x80x9cstructuredxe2x80x9d.
xe2x80x9cRandom packingxe2x80x9d means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
xe2x80x9cStructured packingxe2x80x9d means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal or woven wire screen stacked in layers or as spiral windings.
The term xe2x80x9csurface area densityxe2x80x9d means the surface area of the structured packing per unit volume of the structured packing, and usually is expressed in terms of m2/m3 of the volume occupied by the packing.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
There are many processes for the separation of air by cryogenic distillation into its components (i.e., nitrogen, oxygen, argon, etc.). A typical cryogenic air separation unit 10 is shown schematically in FIG. 1. High pressure feed air 1 is fed into the base of a high pressure column 2. Within the high pressure column, the air is separated into nitrogen-enriched vapor and oxygen-enriched liquid. The oxygen-enriched liquid 3 is fed from the high pressure column 2 into a low pressure column 4. Nitrogen-enriched vapor 5 is passed into a condenser 6 where it is condensed against boiling oxygen which provides reboil to the low pressure column. The nitrogen-enriched liquid 7 is partly tapped 8 and is partly fed 9 into the low pressure column as liquid reflux. In the low pressure column, the feeds (3,9) are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components. Structured packing 11 may be used to bring into contact the liquid and gaseous phases of the oxygen and nitrogen to be separated. The nitrogen-rich component is removed as a vapor 12, and the oxygen-rich component is removed as a vapor 13. Alternatively, the oxygen-rich component can be removed from a location in the sump surrounding reboiler/condenser 6 as a liquid. A waste stream 14 also is removed from the low pressure column. The low pressure column can be divided into multiple sections. Three such sections with structured packing 11 are shown in FIG. 1 by way of example.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils (or corrugated mesh cloths) stacked vertically. These foils may have various forms of apertures and/or surface texture features aimed at improving the heat and mass transfer efficiency. An example of this type of structured packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
The separation performance of structured packing often is given in terms of height equivalent to a theoretical plate (HETP). The term xe2x80x9cHETPxe2x80x9d means the height of packing over which a composition change is achieved which is equivalent to the composition change achieved by a theoretical plate. The term xe2x80x9ctheoretical platexe2x80x9d means a contact process between vapor and liquid such that the existing vapor and liquid streams are in equilibrium. The smaller the HETP of a particular packing for a specific separation, the more efficient the packing because the height of packing being utilized decreases with the HETP.
U.S. Pat. No. 4,836,836 (Bennett et al.) teaches the use of structured packing in cryogenic distillation wherein the power benefits relative to the use of distillation trays is discussed. The teachings of this patent can be applied to all of the packed sections of an air separation plant, although it is most useful in those sections separating argon and oxygen.
U.S. Pat. No. 5,613,374 (Rohde et al.) teaches the use of packing with a surface area density greater than 1,000 m2/m3 as optimal for the low temperature separation of air using at least one rectification column. While the most commonly available structured packings have a surface area density in the broad range of 125-750 m2/m3, Rohde et al. teaches a preferred range of 1000-1500 m2/m3. Although structured packing of such high surface area density could provide an advantage of high mass transfer efficiency leading to a reduction in the heights of distillation columns, there also are several disadvantages. First, the additional surface area would increase the cost of the packing. Second, the high pressure drop associated with the high surface area density would lead to a reduction in capacity, which would result in a significant increase in the diameter of the distillation columns used for this separation, which would further increase the cost of the system. Finally, the increased diameter, together with the high surface area density, also would limit severely the ability of distillation columns to operate in a turndown mode because of the unavailability of enough liquid during turndown to keep the large surface area of the packing well wetted. The ability to turndown the production capacity of a plant without losing efficiency is a critical requirement of most modern distillation plants.
U.S. Pat. No. 5,100,448 (Lockett et al.) teaches the use of variable surface area density packing within a single distillation column of constant diameter. Different sections within a column can be under very different loading conditions in terms of vapor and liquid velocities, especially if there are vapor or liquid draws or feeds in between the sections. If the surface area density of the packing used in all sections is the same, then the column will be unevenly loaded relative to the maximum capacity in each section. Because of this, the ability to operate the plant in turndown mode can be severely limited. The suggested remedy is to change the surface area density of the packing within various sections (or within one section) of the column while maintaining the diameter constant for the purpose of obtaining uniform loading within a column. The purpose is not to optimize the packing, but to obtain uniform loading in the column. Also, if the column diameter is changed between sections, as is frequently the case, then there is no need to do that which is taught by Lockett, et al.
U.S. Pat. No. 5,419,136 (McKeigue) teaches an analogue of Lockett et al. McKeigue addresses the same situation of multiple packed sections within a constant diameter column which have very different loadings if the same packing is used in all sections. In this situation, the ability of the column to operate in a turndown mode will be limited as in Lockett et al. McKeigue""s remedy is to vary the xe2x80x9ccrimp anglexe2x80x9d within sections and/or within subsections of a section. [The term xe2x80x9ccrimp anglexe2x80x9d as defined in McKeigue is the angle that the corrugations make with the vertical. This has a simple relationship to applicants"" xe2x80x9ccorrugation anglexe2x80x9d (xcex1) in the current invention, wherein the corrugation angle is measured relative to the horizontal. Thus, the xe2x80x9ccrimp anglexe2x80x9d in McKeigue is equal to 90xc2x0 xe2x88x92xcex1.] McKeigue""s purpose is to obtain uniform loading within a column, not to optimize the packing. Also, when the column diameter is changed between sections, as is frequently the case, then there is no need to do that which is taught by McKeigue.
U.S. Pat. No. 5,644,932 (Dunbobbin et al.) teaches the use of two different corrugation angles and xe2x80x9ccrimp anglesxe2x80x9d (xe2x80x9cincluded anglesxe2x80x9d in applicants"" terminology) within two packed sections inside a distillation column, wherein the sections have very different vapor and liquid flow rates. The motivation behind Dunbobbin et al. is similar to that in Lockett et al. and McKeigue, namely to design the packings in two sections such that the hydraulic loading capacity of each packing is approached equally. To this end, a new dimensionless parameter xe2x80x98sxe2x80x99 is introduced which is defined as the product of the vapor-liquid interfacial stress and the liquid film thickness divided by the surface tension of the liquid. This xe2x80x98sxe2x80x99 parameter needs to be maintained within a narrow range. However, very broad ranges of the corrugation and included angles are claimed to yield xe2x80x98sxe2x80x99 within the narrow range.
Apertures have been used to improve the efficiency of structured packing. For example, Meier (U.S. Pat. No. 4,296,050) and Huber (U.S. Pat. No. 4,186,159) both state that the apertures in their packing elements are approximately 4 mm in diameter. Most of the major vendors make structured packing with apertures in the range of 4-5 mm. Their apertures occupy not more than 5 to 20% of the total surface area of the element or plate. Also, their corrugations are disposed at an angle of 15xc2x0 to 60xc2x0 relative to the vertical, or 30xc2x0 to 75xc2x0 relative to the horizontal (i.e., xcex1=30xc2x0 to 75xc2x0).
U.S. Pat. No. 4,950,430 (Chen et al.) teaches the use of apertures in the range of 1-2 mm, with several other qualifiers in terms of hole spacing, etc. for an improved packing relative to Meier or Huber. For example, the spacing between the apertures is no greater than 5 mm, and the apertures occupy not more than 20% of the total surface area of the element or plate. In the preferred form; the apertures are round holes, but non-round holes may be used, including ovals, oblongs, elliptical holes, and triangular holes, rectangular holes, narrow slit-type holes, and the like.
U.S. Pat. No. 5,730,000 (Sunder, et al.) and U.S. Pat. No. 5,876,638 (Sunder, et al.) disclose a corrugated structured packing element which preferably has a plurality of apertures throughout the element. The open area of the element may be in the range of 5 to 20% and preferably in the range of 8 to 12% of the total area of the element. The surface area density of the element is preferably in the range of 250-1500 m2/m3, with a most preferred range of 500-1000 m2/m3. The apertures are circular and have a diameter in the range of 1-5 mm, with a preferred range of 2-4 mm. Alternatively, the holes in the packing are not circular, but have an xe2x80x9cequivalent diameterxe2x80x9d (calculated as four times the area divided by the perimeter) within the stated ranges of the diameter of the circular apertures. The corrugation angle (xcex1) is in the range of 20xc2x0-70xc2x0, with a preferred range of 30xc2x0-60xc2x0, and is most preferably 45xc2x0. These patents teach the use of a bidirectional surface texture in the form of fine grooves in crisscrossing relation applied on the surface of the corrugated plates of the packing element.
PCT/EP 93/00622 (WO 93/19335) (Kreis) discloses a corrugated structured packing having a surface area density in the range of 350-750 m2/m3, and a surface area density up to 1200 m2/m3 or higher for certain applications. Apertures in the corrugated elements may be holes, slits or slots. The apertures occupy 5 to 40%, and preferably around 15 to 20%, of the total surface area of the element or plate.
Different surface textures also have been used to improve the efficiency of structured packing. For example, as U.S. Pat. No. 5,454,988 (Maeda) discloses a packing element having plural continuous, adjacent, meandering, concave/convex channels formed on the surface of a sheet-like base. Numerous other examples of surface texture are found in the prior art, such as in EP 0 337 150 A1 (Lockett).
In contrast to the prior art, the motivation behind the current invention is to optimize the packed sections of a distillation plant by simultaneously varying a plurality of independent parameters (e.g., at least four parameters) in order to minimize the overall cost of the system, without a narrow focus on a single criterion such as height or loading.
It is desired to have a structured packing that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It is further desired to have a structured packing of the corrugated type which will be optimal for cryogenic distillation, especially for separating and purifying the components of air, such as oxygen, nitrogen and argon.
It is still further desired to have a structured packing of the corrugated type which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have an optimal design of a structured packing that operates in an optimal manner, which will result in an air separation process more efficient and/or less expensive per unit quantity of product produced.
It is still further desired to have a more efficient air separation process utilizing an optimal structured packing which is more compact and efficient than the prior art.
It also is further desired to have a method of installing a structured packing in an exchange column which affords better performance than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior to provide better and more advantageous results.
The present invention is an optimal corrugated structured packing, which may be used in one or more sections of an exchange column for exchanging heat and/or mass between a first phase and a second phase in a process, such as cryogenic air separation. The invention also provides methods for installing such a packing in an exchange column to provide optimum performance. In addition, the invention includes processes wherein liquid-vapor contact or liquid-liquid contact are established by at least one structured packing of the type taught herein.
In one embodiment, the structured packing has a surface area density in the range of about 350 m2/m3 to about 800 m2/m3, and includes a plurality of corrugated plates disposed in vertically parallel relation. Each plate has at least one aperture and a plurality of regularly spaced and substantially parallel corrugations disposed in crisscrossing relation to the corrugations of an adjacent plate. The apertures have an equivalent diameter less than about 4 millimeters but greater than about 2 millimeters. The corrugations have a corrugation angle (xcex1) relative to horizontal in the range of about 35xc2x0 to about 65xc2x0. Each corrugation, when approximated to be a substantially triangular cross-section, has an included angle (xcex2) defined by two sides of the corrugation in the range of about 80xc2x0 to about 110xc2x0.
In one variation, the structured packing also includes a surface texture applied on at least a portion of the surface of at least one corrugated plate. The surface texture may be in the form of horizontal striations. Alternately, the surface texture may be a bidirectional surface texture in the form of fine grooves in crisscrossing relation.
In another variation, the apertures create open area in each plate in the range of about 5% to about 20% of the total area of the plate. Alternately, the apertures create open area in each plate in the range of about 8% to about 12% of the total area of the plate.
In yet another variation, the corrugations have a root radius (r) in the range of about 0.1 millimeters to about 3.0 millimeters. Alternately, the root radius may be in the range of about 0.3 millimeters to about 1.0 millimeter.
In another embodiment, the structured packing has a surface area density of about 500 m2/m3 to 675 m2/m3, and includes a plurality of corrugated plates disposed in vertically parallel relation. Each plate has at least one aperture and a plurality of regularly spaced and substantially parallel corrugations disposed in crisscrossing relation to the corrugations of an adjacent plate. The apertures have an equivalent diameter less than about 4 millimeters but greater than about 2 millimeters. The corrugations have a corrugation angle (xcex1) relative to horizontal in the range of about 40xc2x0 to about 60xc2x0. Each corrugation, when approximated to be a substantially triangular cross-section, has an included angle (xcex2) defined by two sides of the corrugation in the range of about 90xc2x0 to about 100xc2x0.
In one variation, the structured packing also includes a surface texture applied on at least a portion of the surface of at least one corrugated plate. The surface texture may be in the form of horizontal striations. Alternately, the surface texture may be a bidirectional surface texture in the form of fine grooves in crisscrossing relation.
In another variation, the apertures create open area in each plate in the range of about 5% to about 20% of the total area of the plate. Alternately, the apertures create open area in each plate in the range of about 8% to about 12% of the total area of the plate.
In yet another variation, the corrugations have a root radius (r) in the range of about 0.1 millimeters to about 3.0 millimeters. Alternately, the root radius may be in the range of about 0.3 millimeters to about 1.0 millimeter.
Another aspect of the present invention is an exchange column for exchanging heat and/or mass between a first phase and a second phase, the exchange column having at least one structured packing as in any one of the embodiments or variations described above.
Yet another aspect of the present invention is a process for cryogenic air separation comprising contacting vapor and liquid counter-currently in at least one distillation column containing at least one mass transfer zone wherein a liquid-vapor contact is established by at least one structured packing as in any one of the embodiments and variations described above.
Still yet another aspect of the present invention is a process for exchanging mass and/or heat between two liquids, comprising contacting said liquids in at least one exchange column wherein liquid-liquid contact is established by at least one structured packing as in any of the embodiments and variations described above. In one variation of this aspect of the invention, the liquids flow co-currently in the exchange column. In another variation, the liquids flow counter-currently in the exchange column.
Another aspect of the present invention is a packed section in an exchange column, which includes: a first layer of structured packing; and a second layer of structured packing located below the first layer of structured packing, wherein the second layer is rotated at an angle relative to the first layer. The structured packing in the first and second layers may be any one of the embodiments and variations described above. In one variation of this aspect of the invention, the angle is between about 0xc2x0 and 90xc2x0.
The present invention also includes a method of installing the structured packing in an exchange column comprising multiple steps. The first step is to provide an exchange column. The second step is to provide a structured packing having a surface area density in the range of about 350 m2/m3 to about 800 m2/m3, comprising a plurality of corrugated plates disposed in vertically parallel relation, each plate having at least one aperture and a plurality of regularly spaced and substantially parallel corrugations disposed in crisscrossing relation to the corrugations of an adjacent plate, wherein the apertures have an equivalent diameter of less than 4 millimeters but greater than about 2 millimeters, and wherein the corrugations have a corrugation angle (xcex1) relative to horizontal in the range of about 35xc2x0 to about 65xc2x0 and each corrugation, when approximated to be a substantially triangular cross-section, has an included angle (xcex2) defined by two sides of the corrugation in the range of about 80xc2x0 to about 110xc2x0. The final step is to install the structured packing in the exchange column.
Another embodiment of the method of installing is similar to the method described above, except that the second step utilizes a structured packing such as that described in the second embodiment (xe2x80x9canother embodimentxe2x80x9d) above, rather than the structured packing of the first embodiment (xe2x80x9cone embodimentxe2x80x9d).